[Etiological aspects of orthostatic hypotension].

Orthostatic hypotension (OH) must be distinguished from supine hypotension made worse by standing up and, in particular, from vasovagal syncope. At first approximation, asympathicotonic invariable pulse OH virtually always related to an organic lesion of the baroreflex arch must be distinguished from variable pulse OH which is usually functional and may also be due to organic lesions with exclusive or predominant sympathetic system disorders. In case of doubt, it may be useful to measure palmar and plantar sympathetic potentials. The principal causes of variable pulse OH are therapeutic drugs, absolute or relative hypovolaemia, endocrine diseases (adrenal insufficiency, phaeochromocytoma), spinal quadriplegia and two congenital diseases including dopamine beta-hydroxylase deficiency. In Guillain-Barré syndrome, diabetes and alcoholism, the OH pulse may be variable or invariable. The main causes of asympathicotonic OH are ageing, post-prandial period, certain infections (e.g. tabetic neurosyphilis, botulism, EBV and HIV infections), a few systemic diseases and isolated neurological diseases. Among the systemic diseases responsible for OH are diabetes, alcoholism and chronic liver diseases of other causes, porphyria, lead poisoning, Biermer's disease, amyloidosis, several connective tissue diseases, including systemic lupus erythematosus, and some cancers associated or not with Lambert-Eaton syndrome. Among isolated neurological diseases are the familial diseases described by Riley and Day, multisystem atrophies (first described by Shy and Dager) and pure peripheral dysautonomia. To differentiate the latter from an incipient Shy-Dager syndrome, it may be helpful to use pharmacological tests: plasma catecholamine levels measurements in supine position, and clonidine test with repeated growth hormone assays in upright position.